The beauty blogger - who has 12.4 million subscribers on YouTube and 12.9 million followers on Instagram - went on to explain how she began her transition at the age of six when she grew her hair and began to wear girls' clothes.

"My mum knew immediately that I either was going to be gay or a different type of story. And it turned out to be a different type of story," she said, adding her mum had been "convinced" during her pregnancy she was having a girl.

Nikkie, who began sharing videos of her make-up tutorials online at 14, also spoke openly about her transition, saying she began hormone treatment at the same time.

The YouTube mega star known best as NikkieTutorials explained she transitioned on YouTube. Picture: Instagram / NikkieTutorials

"By the time I was 19, I fully transitioned," she said. "I transitioned while on YouTube."

It's not clear how much her fiance Dylan, who proposed to the star during a trip to Italy in August, knew as Nikkie only briefly touched on their relationship in the video, saying she "wished she told him sooner".

A post shared by NikkieTutorials (@nikkietutorials) on Dec 25, 2019 at 3:01pm PST

While her raw and honest video has been met with applause online, Nikkie explained the blackmail plot forced her hand.

"I have always wanted to share this side of my story with you, I just wanted to do it under my conditions. But apparently we live in a world where people hate on others who are truly themselves," she said.

"I have been blackmailed by people that wanted to 'leak' my story to the press. At first it was frightening to know there are people out there that are so evil that they can't respect someone's true identity. It is vile and it is gross."

Nikkie said the people threatening her had accused her of "lying" to fans and said she didn't want to "tell her truth".

"To the people who tried to blackmail me and thought they could mess up my life, this one's for you," she said, giving a hand gesture to the camera.

She had a message for her blackmailers in the clip. Picture: Instagram / NikkieTutorials

Nikkie rose to fame when her video "The Power of Makeup" went viral in May 2015 when she was 21. It showed her applying beauty products to one half of her face while leaving the other completely natural.

"I filmed it and I almost never uploaded it because I hated it," she told news.com.au last year. "It just didn't feel right. I thought, 'Nobody is going to get this'."

However, her boyfriend at the time convinced her to post the footage, saying he thought people would "resonate" with it.

She previously told news.com.au she almost didn’t share the video that made her famous. Picture: Instagram / NikkieTutorials

"I remember uploading it and it did so bad, I said to him, 'See, I never should have uploaded this, it's horrible'."

The clip has since been viewed more than 40 million times and launched her hugely successful career in the beauty industry. She was named Marc Jacobs' first artistry adviser and is reportedly worth more than $US6 million ($A8.5 million). She was even listed among Forbes "top influencers" in 2017.